Semiconductor core-level to valence-band maximum binding-energy differences: Precise determination by x-ray photoelectron spectroscopy

Angle-resolved core-level and valence-band x-ray photoelectron spectroscopy (XPS) data for GaAs(110), Ge(110), and Ge(111) surfaces are analyzed to determine core-level to valence-band maximum binding-energy differences to a precision of the order of the room-temperature thermal energy. A method for markedly improving the precision with which the position of the valence-band maximum in XPS data can be located is presented. This method is based on modeling the XPS valence-band spectrum in the vicinity of the valence-band maximum by an instrumentally broadened theoretical valence-band density of states and fitting this model to the experimental data by using the least-squares method. The factors which influence the attainable precision for determining core-level to valence-band maximum binding-energy differences are quantitatively discussed. These factors include the presence of occupied surface states, band bending, surface chemical shifts, background effects associated with inelastic processes, instrumental line shape, and spectrometer calibration accuracy. The spin-orbit-split components of the Ga, As, and Ge $3d$ core lines are resolved and binding energies of these components, measured relative to the valence-band maxima in GaAs and Ge, are reported.